1. Field of the Invention
The present invention relates to a method of manufacturing a piezoelectric vibrating reed, a piezoelectric vibrating reed, a piezoelectric vibrator having a piezoelectric vibrating reed, an oscillator, an electronic apparatus, and a radio-controlled timepiece.
2. Description of the Related Art
In recent years, piezoelectric vibrators using crystal or the like are used in mobile phones or portable information terminals as a time source, a control signal timing source, a reference signal source, and the like. Various piezoelectric vibrators are available as such kinds of piezoelectric vibrators, and a piezoelectric vibrator having a tuning fork type vibrating reed is also known as one of the piezoelectric vibrators. This piezoelectric vibrating reed has a pair of vibrating arms disposed in parallel and a base portion which integrally fixes base end sides of the pair of vibrating arms, and vibrates at a predetermined resonance frequency in a direction of moving closer to or away from each other.
For example, a method of manufacturing a piezoelectric vibrating reed is disclosed in JP-A-2002-261557, JP-A-2004-120351, and JP-A-2009-88753. Here, patterning of the outer shape of a piezoelectric vibrating reed will be specifically described. First, a metal film is formed on a wafer made of crystal, for example, and then a resist film is formed on the metal film. Then, using a photolithographic technique, the resist film is patterned by exposure and development performed through a light shielding film pattern of a photomask. As a result, a resist pattern based on the outer shape of the piezoelectric vibrating reed is formed. Then, the metal film is etched using the resist pattern as a mask. As a result, a metal film pattern is formed in which the metal film other than a region protected by the resist pattern has been selectively removed. Then, by etching the wafer using the metal film pattern as a mask, the wafer other than a region protected by the metal film pattern is selectively removed. Thus, the outer shape of the piezoelectric vibrating reed can be formed.
Moreover, at the time of exposure described above, contact exposure may be adopted in which exposure is performed using a negative resist film in a state where the resist film and a photomask are in close contact with each other. In this contact exposure, it is possible to reduce the amount of light entering between the resist film and the photomask because the resist film and the photomask are in close contact with each other. As a result, since the resolution at the time of exposure is improved, the resist pattern can be formed with high dimensional accuracy.
In the case where the resist film is patterned by the contact exposure described above, however, a foreign matter may enter between the photomask and the resist film. As a result, there is a possibility that damage, such as a crack, will occur in an outer shape equivalent region which is equivalent to the outer shape of the piezoelectric vibrating reed in the photomask. In the damaged portion, the transmittance of light at the time of exposure is decreased compared with that in a non-defective portion with no damage. Accordingly, the photoresist film of the region equivalent to the damaged portion is also affected by the damage. That is, the same damage as in the outer shape equivalent region is also transferred to the resist pattern. Accordingly, since the metal film is abnormally etched due to etchant permeating into the damaged portion of the resist pattern when forming a metal film pattern, there is a problem that the metal film pattern is difficult to form in a desired shape. As a result, at the time of subsequent patterning of a wafer, the wafer is abnormally etched. In this case, there is a problem that the obtained shape is defective or recessed (so-called crystal defect) since forming the piezoelectric vibrating reed in the desired outer shape is not possible. A photomask is repeatedly used by washing it after exposure. However, once the photomask is damaged, it is not possible to manufacture a non-defective piezoelectric vibrating reed in the damaged portion.
For this reason, electrical properties, such as a resonance frequency, are tested in the final step in order to sort out defective piezoelectric vibrating reeds and non-defective piezoelectric vibrating reeds. However, depending on the position or the size of a crystal defect, defective piezoelectric vibrating reeds may not be correctly sorted out. In this case, if a defective piezoelectric vibrating reed is distributed in the market and used as a product, there is a problem that the piezoelectric vibrating reed easily breaks with a place of a crystal defect as a starting point by impact, fatigue, or the like.